Composite materials which comprise an organic resin and a filler have been known and used for a number of years. For example, there has been a need to find materials that exhibit the look and feel of natural wood. One reason for this need relates to efforts to conserve the limited supply of natural wood for construction purposes from the world's forest. Another reason is that certain composite materials can exhibit properties that are superior to natural wood in certain respects. For example, it is possible to formulate composite materials into synthetic wood that has enhanced moisture resistance.
In addition to the general cost and difficulty associated with wholesale use of wood products in construction, much of the wood in general board fabrication is wasted material. A substantial amount of sawdust is created together with pulp materials such as branches or the like as the lumber producers endeavor to transform a harvested tree into a collection of elongated boards and planks for use in fabrication of structures. Attempts have thus been made to manufacture products as substitutes for virgin wood utilizing wood fiber or particles together with various binder materials. Such products have become available and are generally known as “fiber board” or “particle board.” In addition, the use of wood particles or fibers together with plastic binders have created so-called plastic wood. As a result, cellulose and related materials are highly desirable material for use in composites in general, and in composites intended for use as wood substitutes in particular.
One problem associated with manufacture and effectiveness of such composite materials is the ability to strongly bind the cellulosic fibers and the thermoplastic binder. Adhesion stability between the filler and the resinous mixture has been recognized as a source of degradation and failure of these materials for nearly as long as such materials have been known. As reported in U.S. Pat. No. 5,981,067, one solution to this problem involves enhancing polymer-fiber compatibility, that is, the tendency of the polymer and fiber to mix and/or adhere to one another. U.S. Pat. No. 5,120,776, which is incorporated herein by reference, teaches cellulosic fibers pretreated with maleic or phthalic anhydride to improve the bonding and dispersibility of the fiber in the polymer matrix. Also of relevance in this regard is Maldas and Kokta, “Surface modification of wood fibers using maleic anhydride and isocyanate as coating components and their performance in polystyrene composites”, Journal Adhesion Science Technology, 1991, pp. 1-14.
While maleic anhydride-based coupling agents have been suggested for use in making composite materials, there has been a decided absence of success in the use of such materials in commercial applications.
Commercial applications of composite compositions frequently involves the shaping of such compositions by molding, extrusion or the like. In order for such operations to be practically effective in a commercially competitive environment, it is necessary that such processes be carried out at a relative high rate and with a minimum of operational problems. Toward this end, the use of additives in the composite composition to aid in the processing thereof are practically essential. One commonly used processing aid is a lubricant or release agent, which allows the effective processing of such composites at commercially acceptable speeds. Metal stearates, and in particular zinc stearate, are frequently used in lubricant packages for composites involving thermoplastic polymers and cellulosic filler. See for example U.S. Pat. No. 6,180,257 B1 (col., 2, 11. 26-28).